TERRACES. 



Terraces and beaches form, perhaps, the most important feature of surface 

 geology ; and, as I have directed my attention chiefly to these, I shall go into more 

 details as to their nature and characteristics. 



It is hardly necessary to say that, though the term terrace applies to any level- 

 topped surface, with a steep escarpment, whether it be solid rock or loose materials, 

 it is only the latter kind which are treated of in this paper; for I shall describe 

 only those terraces which have been formed since the drift period — not even those 

 which may be unconsolidated in the tertiary strata. 



Terraces are of three kinds : — 



1. River Terraces. 



These are the most perfect of all, and are found along the shores of almost all 

 rivers; but especially those passing through hilly countries, and forming narrow 

 basins with a succession of gorges. 



River terraces may be subdivided into four varieties, differing in position, and 

 probably, also, in their mode of formation. 



1. The Lateral Terrace. — This is the ordinary terrace, which we meet along the 

 banks of a river, often many miles in length, and sometimes even miles in width. 



2. The Delta Terrace. — This occurs at the mouths of tributary streams, and was 

 most obviously a delta of the tributary; but, as the waters sunk, the delta was left 

 dry, and the tributary cut a passage through it, so as to form a terrace of equal 

 height on opposite banks. 



3. The Gorge Terrace. — This occurs either above or below the gorges of a stream, 

 and is intermediate between the lateral and delta terraces, graduating into both. 



4. The Glacis Terrace. — This is not level topped, but slopes gradually both ways 

 from its axis — on the side next the stream much more rapidly than on the other. 

 Outwardly it resembles the glacis of a fortification, and hence the name. It is 

 usually found in alluvial meadows, and might, perhaps, be regarded as merely the 

 uneven surface of a lateral terrace, as it is seldom more than a few feet high. But 

 in some of the high valleys of the Alps, I found broad terraces sloping very rapidly 

 towards the stream to its very brink, as well as in the direction of the currents, 

 and Mr. Darwin describes the same kind of terrace in the high valleys of the 

 Andes. Such terraces, then, I should regard as the true type of the glacis terrace, 

 rather than those undulations of surface which we see in alluvial meadows. 



2. Lake Terraces. 



These scarcely differ from the lateral terraces of rivers. Indeed, many small 

 lakes, and even some of the larger ones, appear to have been merely expansions of 

 rivers, such as are now seen in great numbers in the basin of the Upper Mississippi, 

 west and southwest of Lake Superior. (See Nicollet's Map.) These were formerly 

 retained by barriers at a higher level when the terraces were formed, and, as those 

 barriers have been worn away, the terraces have been left on their borders. 



